It is well-known that a photographic film coated with hydrophilic colloid layers at one or two sides of the undercoat, e.g. a polyester undercoat, has a low conductivity due to the electric-insulating properties and becomes electrostatically charged by friction with dielectric materials and/or contact with electrostatically chargeable transport means, e.g. rollers. The charging occurs particularly easily in a relatively dry atmospheric environment, and especially with rapidly moving mechanical transport systems. The electrostatical charge that is accumulated may cause various problems due to the fact that it cannot be discharged gradually. As a consequence e.g. partial exposure of the photosensitive silver halide emulsion layers of the photographic material after an abrupt discharge may occur before development. This partial exposure results in the formation of dot-like or branch-like or feather-like spots after development of the photographic material.
In practice the photographic material is subjected to frictional contact with other elements during manufacturing, e.g. during a coating or cutting stage, and during use, e.g. during image-processing. Especially in the reeling-up or unreeling of dry photographic film in a camera high friction may build up, resulting in electrostatic charges that may attract dust or cause sparking. In unprocessed photographic silver halide emulsion materials sparking causes undesirable exposure marks and degrades the image quality.
These disturbing phenomena however cannot be observed prior to development. As this phenomenon is very irreproducible, difficulties arise for the quality control department to evaluate said photographic material.
In order to reduce electrostatic charging of a photographic material comprising a hydrophobic resin undercoat layer or support and at least one hydrophilic colloid layer on at least one side of said support without impairing its transparency it is known to apply coatings which are formed of or incorporate ionic compounds such as antistatic high molecular weight watersoluble polymeric compounds having ionic groups at frequent intervals in the polymer chain [ref. e.g. Photographic Emulsion Chemistry, by G. F. Duffin, --The Focal Press--London (1966)--Focal Press Limited, p. 168, U.S. Pat. No. 4,301,240].
Especially preferred antistatic compositions have been described in U.S. Pat. No. 4,610,955. These compositions comprise a hydrophilic binder, a surface active polymer having polymerized oxyalkylene monomers and an inorganic salt of organic tetrafluoroborates, perfluoroalkylcarboxylates, hexafluorophosphates and perfluoroalkyl carboxylates, said fluorinated surfactants leading to a good coating quality of the hydrophylic layers.
To minimize the electrostatic charge properties of photographic materials, especially the tribo-electrical charging causing electrostatical discharges and mechanical faults by transporting, it has been proposed according to EP 319 951 to use in the hydrophilic colloid layer a combination of three surfactants viz. an anionic fluorinated surfactant, a nonionic oxyalkyl compound and a nonionic oxyalkyl compound containing fluorine atoms.
Nevertheless a remaining problem is the preservation of the antistatic properties during storage of the photographic material for a long time after manufacturing, especially when said storage takes place in severe circumstances as e.g. at high temperature and high relative humidity.
A solution for the preservation problem of the antistatic properties may be offered by the coating of a thicker antistress layer with an increased amount of antistatic agents, e.g. polyoxyalkylene polymers. Although these increased amounts have the advantage of giving rise to more surface glare after processing, an inadmissable contamination or sludge formation in the coating step and, after exposure and development, may occur in the processing solutions. Moreover a thicker hydrophilic layer may retard the processing and drying velocity. This is obviously contradictory to the trend to develop rapid processing systems characterized by films with thin coating layers.